[en] Aphid alarm pheromone—the volatile sesquiterpene (E)-β-farnesene (Eβf) in most species—is released in response to predation and other stresses and typically causes nearby aphids who receive the signal to cease feeding, drop from their host plant, and disperse. Because aphid alarm pheromone confers apparent fitness benefits on recipients while its production and release likely entail costs for the emitting aphid, it could be adaptive for aphids to regulate their Eβf production in response to variation in the social environment. To explore this possibility we compared the production of Eβf by Acyrthosiphon pisum (Harris) individuals reared from first-instar larvae to the adult stage in isolation to that of individuals reared among conspecifics or among individuals of a different aphid species, Myzus persicae (Sulzer). Levels of EβF produced in each treatment were assayed by GC-FID quantification of EβF in volatiles collected from crushed aphids. Production of EβF by A. pisum reared in isolation (14.4ng/aphid) was significantly lower than that of aphids reared in a colony of conspecifics (49.1ng/aphid), reared in a M. persicae colony (31.5ng/aphid) or reared among conspecifics of another aaphid clone (52.7ng/aphid). Though A. pisum individuals in our experiments produced less EβF when reared among M. persicae than among conspecifics, this difference was not statistically significant. In a separate experiment we reared A. pisum individuals in isolation and exposed them to the odor of conspecifics. Under these conditions, EβF production was similar to that of aphids reared among conspecifics, suggesting that aphids use volatile cues to assess their social environment and regulate their production of alarm pheromone accordingly.